1. Field of the Invention
This invention relates generally to circular saw blades and more particularly to a low noise, high speed circular saw blade.
2. Description of the Prior Art
There are several types and designs of saw blades for cutting different types of material. These saw blades have in common a plurality of teeth spaced about the outer periphery of the blade, with the teeth being provided with suitable cutting edges. Usually, most saw blades also include a plurality of expansion slots in the outer edges of the saw blades in order to prevent heat warpage of the blades.
An expansion slot may be formed in one of three directions. It may be formed in a radial direction (i.e., directly aligned with the center of the blade); it may be formed at a so-called "positive" angle; or it may be formed at a "negative" angle. A positive angle is an angle such that the bottom of the slot trails a line extending from the center of the blade to the outer edge of the slot, relative to the direction of rotation of the saw blade. In contrast with a positive angle, the bottom of a negative angle slot leads the radial line in the direction of rotation of the saw blade. The angle of an expansion slot is sometimes referred to as the "hook" of the slot, with a radial slot having a 0.degree. hook, and positive and negative angle slots having positive and negative hooks, respectively.
Different types of saw blades and different types of expansion slots are provided for cutting different types of material. One type of saw blade is commonly referred to as a "green wood" saw blade. This is a large diameter saw blade (i.e., from 30 to 100 inches in diameter) and is employed for cutting raw lumber in lumber mills. Because of the large size of these saw blades, green wood saw blades generally are operated at relatively slow rotation speeds of about 500-1200 revolutions per minute (r.p.m.), although some green wood saw blades can be rotated at speeds of up to 3600 r.p.m.. The risk with higher rotation speeds is that the blade can fly apart.
One problem associated with a green wood saw blade is that the cut wood tends to "grow back" or swell up against the blade, and fresh, moist sawdust produced by the cutting action binds in the space between the face of the saw blade and the wood being cut. This causes excessive friction between the blade and the wood, and this can damage the blade. In order to avoid this problem, "green wood" saw blades are provided with wide (i.e., at least 1/4 - 1/2 inch) and deep (i.e., at least 1/4 of the diameter of the blade) expansion slots formed at a negative angle up to 40.degree., with the expansion slots being provided with wide cutting edges or inserts along the trailing edges of the slots. As these saw blades rotate and the sawdust enters the space between the saw teeth and the face of the saw blade, the sawdust comes in contact with the angled cutting edges on the expansion slots and is propelled by centrifugal force along the cutting edges out of the area between the saw blade and the wood cut.
One of the side effects of employing inserts in the expansion slot of a saw blade is that they tend to permit the blade to flop back and forth. This makes such blades undesirable in cutting dry woods. A drawback with employing wide slots is that blades having wide slots cannot be used safely for dry woods. One reason for this is that when dry woods are cut, hard splinters and chips are produced, and these can become wedged into a wide slot in a blade. When this occurs, the splinter or chip wedged in the expansion slot causes the rapidly rotating saw blade to catch the piece of wood and impart a strong force to the wood. This can cause the wood to fly off the cutting table with substantial velocity, which is dangerous. Wet woods do not produce the same types of splinters and chips, so this risk is not present with wet woods. The use of narrow expansion slots in dry wood saw blades substantially eliminates the risk of chips and splinters getting caught in the expansion slots. Because of the foregoing considerations, blades with wide slots and inserts along the slots typically are used exclusively for green woods and generally are rotated at slower speeds. Such blades are used almost exclusively for cutting raw lumber in lumber mills.
Most circular saws employed for cutting lumber after it has been pre-cut into useable sizes in a lumber mill are called "high speed" saws. These saws operate at rotation speeds of about 1700 to 7200 r.p.m., with the most common rotation speed being about 3450-3600 r.p.m. These saws are used in almost all wood cutting operations and are also used for cutting plastics, chipboard, and hardboard, and other similar material. High speed saw blades may have a diameter up to 30 inches but generally the diameter is 20 inches or less. The rotation speeds permissable with such saw blades are inversely proportional to the diameter of the blade. Thus, it is possible to run a 30 inch high speed saw blade at speeds up to about 1725 r.p.m., while a 20 inch high speed saw blade may be operated safely at 3450 r.p.m. and a 14 inch blade may be operated safely at 5000 r.p.m.
Two types of high speed saw blades that are of particular interest in the context of the present invention are known as "cut-off" or "cross cut" saw blades and "rip" saw blades. "Rip" saw blades are designed for cutting lumber along the longitudinal direction or with the grain of the lumber, and when several of these saws are attached together in parallel, the saws are referred to as "ganged" rip saws.
"Cut-off" saw blades are designed for cutting across the grain of the wood.
High speed saw blades, like low speed saw blades, employ expansion slots to prevent blade warpage due to heat buildup in the blade. However, sawdust buildup adjacent the face of the blade is not a problem with high speed saw blades (which generally are employed for cutting dry wood or other such materials), so no special design criteria is necessary for expansion slots in high speed saw blades. Typically, such slots are usually about one inch deep for a 12-30 inch blade and about 3/4 inch for a 6-12 inch blade and are formed at the same positive hook as the teeth of the blade (which may be between +10.degree. and +30.degree.). Occasionally, the expansion slots are formed in a radial direction or at a 0.degree. hook. One of the problems encountered with high speed saw blades is that operation noise levels are practically intolerable to the human ear. The noise of such saws rotating freely may be in excess of 100 decibels when freely rotating and as high as 110-115 decibels during the cutting operation, depending on the type and thickness of the material being cut. Federal authorities have specified a 90 decibel ceiling for such saws, and the saws presently in use are incapable of satisfying this criteria. With conventional dry wood cutting saws, the noise problem becomes significant at speed in excess of 2600 r.p.m.
The present invention provides an improved circular saw blade, wherein operation noise levels of the saw are reduced to acceptable levels.